Conventionally, in a double-pipe heat exchanger of this type, a heat-transfer facilitating body such as an inner fin having dimple-like projections and depressions is inserted between an inner pipe and an outer pipe. The heat-transfer facilitating body produces turbulent flow, thereby it enhances the heat-transfer performance of the heat exchanger (see Patent Document 1 for example).
[Patent Document 1]
Japanese Patent Application Laid-open No. H9-145285 (pages 2 to 4, FIG. 4)
In the conventional structure, however, since the heat-transfer facilitating material such as the inner fin is required in addition to the inner pipe and the outer pipe which constitute a double-pipe, there is a problem that the material cost is higher than a normal double-pipe.
The present invention has been accomplished to solve such a conventional problem, and it is an object of the invention to provide a more inexpensive double-pipe heat exchanger having higher performance without adding a new material other than the inner pipe and the outer pipe, by enhancing the heat-transfer performance only by subjecting the outer pipe to simple working.